by Eleclriclti/ in Motion. 287 



approximation. If, on the contrary, we sink it below, tlie needle 

 turns about, to present its other pole, and is then attracted 

 anew. But if we constrain it to present the same pole as at 

 6rst, the needle is repelled, and the effects are precisely inverse 

 on the right and on the left hand of the wire. 



If instead of transmitting the electrical current across a 

 simple wire, we make it pass through tubes, plates, or other 

 bodies of a sensible breadth, whose surfaces are composed of 

 parallel right lines, we find that all these bodies act on the 

 magnetic needle, as bundles of wires parallel to their length 

 would do ; which proves that the power developed in them by 

 the electrical current is exerted freely through their very sub- 

 stance, and is not weakened by its interposition, as the radiation 

 of heat tlirough hot bodies is enfeebled and intercepted by the 

 interposition of these very bodies. 



Instead of leaving the needle in the preceding experiment at 

 liberty to move, fix it invariably, but render the conjunctive 

 wire mobile, by suspending it on two points ; then it will be 

 the latter which will move towards the needle, or recede from it. 

 In fact, it is a general law of mechanics, that reaction is always 

 equal to action. If the wire attract or repel the needle in 

 certain circumstances, the needle ought in the same circum- 

 stances to attract or repel the wire. This experiment belongs to 

 M. Ampere. 



Now, let us operate no longer with a magnetic needle on the 

 wire placed in its position of mobility, but let us expose it to 

 the magnetic action of the terrestrial globe, which is known to 

 be perfectly similar to that of a coinmou magnet whose poles are 

 very distant. This force will likewise make the conjunctive wire 

 move according to the same laws, at least if it be sufficiently 

 freely suspended, and it will impress on it a determinate direc- 

 tion relative to the plane of the moiidian, just as it would 

 direct any other magnetic body. This result was realized by 

 M. Ampere''. 



* The accuracy of tliis result has been (jiu-stioned by some able pliilo- 

 ioiilic'is ill (his country, on liolli theordiciil ami cxpoi-iirriital i^i'oinul.-. 



\1. \in|)(ic'> 



